The broad long-term objective of this study is to understand the molecular mechanism of olfactory chemoreception in mammals. High level and unique forms of cytochrome P450 have been found in mammalian olfactory mucosa. The abundance of P450 and other biotransformation enzymes accounts for the remarkably high rates of metabolism of numerous inhaled xenobiotics, including odorants, in the olfactory tissue. Expression of multiple forms of olfactory P450 has also been detected in fetal and neonatal animals, which coincides with the early expression of olfactory signal transduction components. In addition, P450s NMa and NMb, two of the major P450s in rabbit olfactory microsomes, have been detected in the microvillus membrane of the sustentacular cells and the extracellular mucus layer in some regions of the chemosensory epithelium, as well as in the Bowman's glands, which implies that these enzymes may have direct access to odorants in the receptor microenvironment and that odorant metabolites may reach the receptors in reasonably short time. It has been speculated that 1) biotransformation of odorant molecules is important for their removal or modification and 2) such "perireceptor events" contribute to the maintenance of the olfactory acuity and, with the formation of one or more metabolites that have different odor quality, may have an effect on the characteristic odor of a compound. The specific aim of this proposal is to examine the effects of P450-catalyzed metabolism on odorant-induced desensitization of signal transduction in isolated cilia preparations. Microsomal or reconstituted purified P450 isozymes will be added to the cilia preparation. The kinetics of odorant-induced changes in the cAMP level and the length of desensitization of odorant-stimulated adenylate cyclase activity in the presence or absence of a P450-dependent odorant removal system will be examined. Thus, the present study intends to fill the gap between biochemical studies of the olfactory biotransformation and the physiology of olfactory chemoreception with a simple reconstituted system containing the signal transduction cascade and the biotransformation enzymes, which simulates the in vivo situation while emphasizing the possible role of biotransformation in odorant clearance. If time permits, additional studies will be performed to examine the possible role of the olfactory-specific P45ONMb in odor signal transduction; this unique P450 is the principal sex steroid hydroxylase in olfactory microsomes and forms an unidentified metabolite from arachidonic acid. Thus, a second aim is to examine the effects of these endogenous substances and their metabolites on the rapid kinetics of odorant-induced changes in the cAMP level in the olfactory cilia preparation. The outcome of these studies may improve our understanding of the molecular mechanism of olfactory sensory disorders such as anosmia.